The present invention is directed to recovery of floatage, mainly oil after a spill. It is directed particularly to skimmers employed for that purpose.
The job of cleaning up oil spills tends to be complex and inelegant. Reclaiming land that has been subjected to spills can include bulldozing the oil-coated soil, rocks, etc. into dump trucks and disposing of the resulting fouled debris, usually in a manner that is not entirely satisfactory environmentally. It may also include spreading straw to absorb the oil and then retrieving the oil-laden straw. This approach is ordinarily quite labor-intensive and far from thorough. Another approach involves using water or steam from high-pressure hoses to drive oil off the rocks and other earth that it covers. Often, a combination of these approaches appears to be required.
Not only are these approaches generally inelegant, but the manner in which spills often occur significantly detracts from their effectiveness. Most require a fair amount of equipment, much of it heavy, such as trucks, holding tanks, pumps, and other equipment that is difficult to move rapidly to a spill site. Moreover, many beaches that must be cleaned are remote from all roads. Some are even on islands that have no roads at all. And the terrain on and near some beaches is so rugged that there is little or no place to put the required heavy equipment.
Because such considerations significantly restrict activities ashore, it may not be feasible to provide for storage of the recovered oil on land near the recovery site. In such cases, another dimension is added to the problem, because activities onshore may have to be coordinated with those of skimming vessels used to recover oil driven from the beach into the sea by, say, high-pressure hoses and then somehow "herded" out to the skimming vessel's operating depth.
Complicating these logistical problems is the fact that recovery becomes more difficult if it is delayed: exposure to the sun for any significant length of time tends to give the oil a tar-like consistency, which makes it hard to remove from rocks and other matter to which it has adhered.
As a practical matter, oil from spills will almost inevitably reach land. For one thing, circumstances often seem to conspire to impose a significant delay between the spill's occurrence and the time when removal personnel are apprised of it. Furthermore, safety considerations dictate that some time elapse between the spill and entry of motorized vessels on the scene: volatiles from the spill can make the vessels' engines receive too rich a fuel mixture.
For all of the reasons mentioned above, however, it is best if as much oil as possible is recovered while it is still out at sea, since recovery at sea is not nearly as complex as recovery on land. But even sea-surface recovery is challenging. Recovery of oil from the sea surface requires an even faster response than recovery from beaches in many cases if beach fouling is to be kept to a minimum.
Additionally, oil recovery at sea has itself tended to require a considerable amount of costly equipment. This results partially from the fact that the rate at which oil can be recovered depends on the rate at which the skimmer can deal with the resultant recovered liquid. If the skimmer is small and its storage capacity is therefore low, a high recovery rate means one of two things: either the skimmer must make frequent trips to shore to off-load the recovered liquid, or other vessels must make frequent trips to and from the skimmer. The frequency of these trips is reduced only at the cost of providing large storage tanks on the skimmer.
One approach that greatly ameliorates this problem is the one represented by skimmers that employ what are known as "dynamic inclined planes" ("DIPs"). The DIP approach, described in detail in U.S. Pat. Nos. 3,716,142 to Bianchi and 3,804,251 to Farrell et al., employs an endless conveyor belt partially submerged in the water so as to provide a lower belt surface that is inclined and travels from a forward position above the surface to a rearward position beneath the water. The rate of belt travel so matches the speed of the skimmer through the water that floatage, such as oil, tends to be submerged by the conveyor belt and travel back along it until the floatage reaches the rear of the belt, where it floats upward into an open-bottom collection tank, from which it is pumped to (typically onboard) storage.
The oil depth thus achievable in the collection tank greatly exceeds that which results from the concentrator booms employed in other approaches, and pumps that draw the oil from the collection tank into the vessel's storage tank thereby tend to draw fluid in which the oil concentration is, on the average, five times as great as that drawn in other approaches. The pumps, pipes, and storage tanks employed in DIP-type skimmers can therefore be much smaller for a given capacity than those that competing approaches require. Moreover, this advantage tends to propagate itself through the processing chain, tending to require less transport tonnage and less decanting and other equipment at the ultimate processing center.
The DIP approach has accordingly enjoyed significant acceptance in a number of applications. Despite its advantages, however, there are a number of applications on which operators have been unable to use it. This is a result of the fact that smaller DIP skimmers do not lend themselves to rapid deployment; over the years it has generally proved true that increases in a DIP skimmers' speed capacity are obtained at the cost of significant increases in size and thus in expense. While the DIP approaches other advantages justify this additional size and cost for high-capacity applications, it has been necessary to settle for low vessel speed when low-capacity applications are involved, and this compromise is not always acceptable. Even in some large spills, moreover, the diffuse oil distribution makes it preferable to use many small skimmers instead of only a few large ones. Throughout its history, therefore, the range of applications in which the DIP skimmer has been used has been limited because of deployment considerations.